Organic photovoltaic (OPV) cells offer potential advantages over traditional inorganic solar cells which include the use of low-cost light-weight materials, compatibility with plastic substrates to yield flexible solar cells, and amenability to inexpensive, low-temperature, reel-to-reel solution-processing techniques. These potential advantages have spurred researchers in recent years to make significant advances in the efficiencies of OPVs up to a current highest power efficiency near 6% for bulk-heterojunction cells [1-6]. Although this is still significantly below the efficiency of single crystal-Si cells, it approaches that of amorphous silicon (a-Si) cells with power efficiencies about 7-10%. At this point it is thought that OPVs will likely become commercially viable [7].
Recent advances in OPVs have utilized new materials for collecting light and transporting charges to the electrodes [8, 9], while others have come from redesigning the cell architecture to more efficiently separate excitons and collect the resultant charges [10-12]. In a bulk-heterojunction (BHJ) solar cell, a donor polymer and an acceptor material are combined in solution and together spin-coated to form a phase-separated blend on the transparent conductive anode, usually tin-doped indium oxide (ITO). Fabrication is completed by depositing a metal such as aluminum as the cell cathode. The BHJ cell design is a major improvement over bilayer cell designs because it allows photogenerated excitons to reach the donor/acceptor interface to form holes and electrons before recombination. Despite the large improvement in efficiency achieved by the BHJ design, one major disadvantage of the BHJ architecture is the inherent disorder in the heterojunction. After the photogenerated excitons separate, charges travel a circuitous route within their respective material (holes in the donor network, electrons in the acceptor network), often in close proximity to the opposite charges, until collection at the electrodes or recombination occurs.
In addition to charge recombination within the active layer reducing efficiency in BHJ cells, charge recombination at the active organic layer/electrode interfaces is also a problem that can erode device efficiency [13]. One reason for this is a poor surface energy match between the organic active layer and the inorganic electrodes. If an acceptable ohmic contact is not made, charges do not pass freely to the electrode, and device performance suffers. Even if contact is good initially, under heat and light, the organic active layer may lose cohesion with the ITO over time, compromising device durability.
Another loss at the electrode/active layer interface arises from the BHJ design having both the donor and acceptor in contact with both electrodes, allowing charges to flow in the wrong direction. That is, it becomes energetically favorable for electrons formed near in proximity to the anode, for example, to travel from the lowest unoccupied molecular orbital (LUMO) of the acceptor network to the ITO anode. Even though the built-in electric field of the device would direct these charges the other way, there is not always a direct pathway for these charges to travel along the electric field in the disordered BHJ [1, 14], and the energetically favorable transfer of electrons from the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to the ITO anode at their interface represents loss via charge recombination [15]. The small bias from these charges “leaking” to the wrong electrode reduces the observed open-circuit voltage (Voc) and power conversion efficiency (ηp) of the device [16].
One way to compensate poor active layer/electrode contact and charge leakage is to insert an interfacial layer that improves contact and only allows charge carriers of the proper type to pass through to the electrode. The performance of BHJ cells having the three-layer structure ITO/donor:acceptor/Al is improved substantially by inserting interfacial layers between the active organic and the electrodes [15]. A thin layer of lithium fluoride (LiF) is deposited on the cathode side, and a thin semiconducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is frequently used on the anode side.
FIG. 6 shows schematically a conventional BHJ solar cell 10 including an ITO anode 12 formed on a glass substrate 11, a PEDOT:PSS layer 13 formed on the ITO anode 12, an active layer 14 of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and PCBM formed on the PEDOT:PSS layer 13, a LiF layer 15 formed on the active layer 14, and an aluminum (Al) cathodal layer 16 formed on the LiF layer 15. For such a BHJ solar cell, the open-circuit voltage Voc=0.82 V, short-circuit current JSC=5.25 mA/cm2, fill factor (FF)=61%, and ηp=2.5% [17]. The role of a very thin layer (about 0.6 nm) of LiF has been explored previously, and it is thought to either protect the organic materials during cathode deposition, modify the work function of the electrode, or form a dipole moment across the junction and leads to increased charge transfer to the electrode [18-20]. The same active layer incorporated into a cell without a LiF layer yields a consistently lower Voc, FF and ηp [18]. Similarly on the anode side, the PEDOT:PSS interfacial layer significantly increases VOC [21, 22] of the cell. The PEDOT:PSS is also reported to enhance device consistency, with fewer cells shorting out when the PEDOT:PSS is used [21].
Despite these advantages of a PEDOT:PSS interfacial layer formed on an ITO anode compared to a bare ITO anode, the PEDOT:PSS is a very corrosive aqueous blend of materials having pH<1 [23-25], and films of PEDOT:PSS on an ITO anode have been shown to corrode the underlying surface. XPS shows that the PEDOT:PSS actually partially dissolves the surface ITO anode with In and Sn diffusing through the organic film [25]. Also, since no strong covalent bonds holding the PEDOT:PSS to the surface, the PEDOT:PSS can undergo dewetting from either the underlying ITO surface or the overlying active layer surface on exposure to heat, leading to catastrophic decreases in device performance [26].
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.